1. Field of the Invention
The invention relates to the reinforcement of pistons for internal combustion engines.
2. Review of the Prior Art
It has been common for many years to manufacture such pistons from light metals such as aluminium, aluminium alloys or magnesium alloys. This has the advantage that the mass and inertia of the piston is reduced. However, many of these metals are not readily able to withstand the conditions encountered in operation. For example, in diesel engines, the temperatures encountered by the crown of the piston may be sufficiently high to crack such lightweight materials. The crown may also be eroded by the combustion gases. In addition, the ferrous piston rings which are commonly carried in piston ring grooves in such pistons are liable to wear the piston ring grooves. Wear can also occur in the piston gudgeon pin bores of such pistons and the strength of aluminium and aluminium alloys may not be sufficient to withstand the operational stresses encountered in other regions such as expansion slots.
For this reason, it has been proposed to reinforce various regions of light metal pistons so that they are better able to withstand such adverse conditions. For example, it has been proposed to provide pistons with ceramic crowns able to withstand more elevated temperatures than the light metals and to provide piston ring grooves with ferrous reinforcements.
Where the piston is manufactured by a squeeze casting process, in which molten piston metal is placed in a mould and is then solidified under high pressure, use has been made of a reinforcement formed of fibres, for example fibres of aluminium oxide, arranged in a uniform mass. Such a reinforcement is placed in the mould before casting and the pressure applied during solidification forces the molten metal into the voids formed by the fibres so that, on solidification, the reinforcement is incorporated into the piston.
It is a problem, however, different regions of a piston require different reinforcements. For example, the piston ring groove region may require reinforcement against wear while the crown requires reinforcement against the effects of elevated temperatures and gas erosion effects.